1. Field of the Invention
The present invention relates to motorcycles. More particularly, the present invention relates to a motorcycle that suspends a pivotally mounted powertrain unit from a rear portion of a body frame, and includes a storage box above the powertrain unit, and further includes a lubrication-oil passage structure for an engine provided with an enclosed crank chamber.
2. Description of the Background Art
Heretofore, a motorcycle has been proposed in which a seat and a storage box are arranged so as to be vertically stacked on each other in a rear portion of a body frame. In addition, a pivotally mounted powertrain unit is suspended from the body frame under the storage box. An engine of the powertrain unit is constructed so that an engine body, a transmission and a rear wheel extend in the fore-and-aft direction of the body frame. An example of such a motorcycle is disclosed in Japanese Patent Laid-Open No. 2003-182667.
In addition, it is known to provide a motorcycle having an engine provided with dry-sump lubrication means that is also provided with an enclosed crank chamber. An example of a motorcycle having this configuration is disclosed in Japanese Patent Laid-Open No. H07-63034.
In Japanese Patent Laid-Open No. 2003-182667, in which a motorcycle is provided with the storage box positioned above the pivotally mounted powertrain unit, when the storage box is extended upward in order to increase a capacity of the storage box, the ground clearance of the seat, which is mounted above the storage box, is increased. As a result, it becomes difficult for a passenger to get on and off the motorcycle. Meanwhile, when the powertrain unit itself is disposed at a lower position in order to increase the capacity of the storage box, the minimum ground clearance of the powertrain unit is reduced. Moreover, when a position of a crankshaft of the engine is lowered downward in order to reduce the overall height of the powertrain unit, an amount of lubricating oil agitated by the crankshaft is increased, resulting in an increase in agitation resistance of the lubricating oil. Further, when it is attempted to restrict the agitation resistance, it is necessary to reduce a volume of the lubricating oil. In any case, a decrease of engine performance occurs.
In an engine in which a crankshaft is freely rotatably supported by journal bearing portions in the crankcase with plain bearings such as metal bearings, in order to achieve further downscaling of the engine, and furthermore, in order to achieve further downscaling of the crankcase, and cost cutting thereof, the engine is configured to forcedly feed lubricating oil to the supporting portion.
However, in such an engine, the lubricating oil passes through between the journal bearing portion and the crankshaft and enters the enclosed crank chamber, the stored oil quantity in the chamber increases, and resistance to agitation by rotation of the crankshaft therefore increases.
The present invention has been created in consideration of such circumstances. A first object of the present invention is to provide a novel motorcycle, in which an improvement is added to a powertrain unit, thereby being adapted to solve the above-described problems, and to make it possible to increase a capacity of a storage box.
A second object of the present invention is to provide a novel lubrication-oil passage structure for an engine, which prevents the lubricating oil from entering the enclosed crank chamber to the extent possible and is thus made capable of solving the above problem.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.